1. Field of the Invention
The present invention relates generally to improved methods for determining fracture parameters of subterranean formations, and more specifically relates to improved methods for determining fluid-loss coefficient, spurt-loss and closure pressure for such formations.
2. Description of the Related Art
It is common in the industry to hydraulically fracture a subterranean oil-bearing formation in order to increase oil production. The success of a hydraulic fracturing treatment often hinges on being able to reasonably estimate the rate at which fluid leaks off from the fracture into adjacent permeable formations. An overestimate of fluid-loss rate can result in the use of excessive pad volumes, leading to increased treatment costs and increased potential for formation damage. More importantly, an underestimate can result in the use of insufficient pad volumes or insufficient fluid-loss control additives, resulting in premature treatment screen-out.
An indirect measurement of the effective fluid-loss coefficient is provided by the "minifrac" or the "minifracture" analysis which is well known in the art. Minifracturing was developed as a pretreatment technique for gaining information on fracture growth behavior. Since its inception, various expansions, modifications, and refinements have increased the applicability of the minifrac analysis. By using the minifrac analysis to analyze the pressure decline during the shut-in period following the creation of a small test fracture, or even a full-scale fracture, parameters such as fracture width and length, fluid efficiency, and closure time may be determined. Of the parameters that may be determined from the minifrac analysis, the most useful, at least for design purposes, has been the effective fluid-loss coefficient. The effective fluid-loss coefficient provided by minifrac analyses has usually provided more reliable estimates of fluid-loss rates and volumes than can be obtained through theoretical calculations based on fluid and formation properties and laboratory measurements of filter cake resistance.
Despite the increased reliability typically associated with fluid-loss coefficients obtained from minifrac tests, the insufficiency of conventional test techniques for naturally fractured formations is becoming known in the art. It is known that these techniques fail to adequately predict formation behavior. For example, sand-out cases have occurred that the conventional analysis and design techniques could not predict.
An attempt was made to solve this problem empirically by developing a correlation based on numerous field cases. Although the correlation has been applied successfully, its use may be limited to the formations for which it was developed. Similar correlations will have to be developed for various naturally fractured reservoirs.
Further, conventional minifrac analyses (see, e.g., U.S. Pat. No. 4,749,038) do not properly address spurt-loss. Spurt-loss is an initial brief period of rapid fluid-loss that has been observed in laboratory experiments. Because of its brevity, it is commonly characterized as an apparent positive intercept on plots of fluid-loss volume versus square root of time plots.
Most minifrac analysis techniques ignore the effect of spurt-loss. The only attempt to consider the effect of spurt-loss was presented by Nolte. Nolte, K.G., "A General Analysis of Fracturing Pressure Decline With Application To Three Models, SPE Formation Evaluation, December, 1986, pages 571-83. Nolte utilized a term .kappa. to account for increased fluid-loss during pumping due to spurt-loss. However, no technique to calculate this factor from pressure decline data was proposed.
The present invention is directed to an improved method of determining the fluid-loss or leakoff coefficient and the spurt-loss of a subterranean formation using a general minifrac analysis.
Accordingly, the present invention provides a new method for determining the spurt-loss and leakoff coefficient of a subterranean formation.